Generally, as opposed to a disposable primary battery, a secondary battery is rechargeable, and is being widely used in electronic devices such as mobile phones, laptop computers, and camcorders, or electric vehicles, and the like. Particularly, a lithium secondary battery has an operating voltage of about 3.6V, and has three times higher capacity than that of a nickel-cadmium battery or a nickel-metal hydride battery being widely used as a power source of an electronic device and a high energy density per unit weight, and thus, its use is on a dramatic upward trend.
A lithium secondary battery mainly uses a lithium-based oxide and a carbon material as a cathode active material and an anode active material, respectively. A lithium secondary battery includes an electrode assembly in which a cathode plate and an anode plate respectively coated with a cathode active material and an anode active material are disposed with a separator interposed therebetween, and an outer casing or a battery case to hermetically receive the electrode assembly together with an electrolyte solution therein.
Meanwhile, a lithium secondary battery may be classified into a can-type secondary battery in which an electrode assembly is embedded in a metal can and a pouch-type secondary battery in which an electrode assembly is embedded in a pouch of an aluminum laminate sheet, based on a case shape.
A secondary battery is generally fabricated through a process of pouring a liquid-state electrolyte or an electrolyte solution in a state that an electrode assembly is received in a case, and sealing the case.
FIG. 1 is an exploded perspective view illustrating construction of a pouch-type secondary battery according to a related art, and FIG. 2 is an assembled view illustrating the pouch-type secondary battery of FIG. 1.
As shown in FIGS. 1 and 2, the pouch-type secondary battery generally includes an electrode assembly 20, and a pouch case 30 to receive the electrode assembly 20 therein.
Here, the electrode assembly 20 has a basic structure of a cathode plate and an anode plate with a separator interposed therebetween, and is received in an internal space formed in the pouch case 30. In this instance, the pouch case 30 may be made up of an upper pouch 31 and a lower pouch 32, and sealing parts are provided along the periphery of the upper pouch 31 and the lower pouch 32 and adhered to each other to hermetically seal the internal space in which the electrode assembly 20 is received.
Meanwhile, at least one cathode tab 21 and at least one anode tab 22 may extend from the cathode plate and the anode plate, respectively. Also, the cathode tab 21 and the anode tab 22 are each joined with an electrode lead 10, that is, a cathode lead 11 and an anode lead 12, and parts of the cathode lead 11 and the anode lead 12 are exposed to the outside of the pouch case 30 to provide an electrode terminal to enable an electrical connection with an external element of the secondary battery, namely, another secondary battery or an external device.
One of the important issues raised in a secondary battery is ensuring safety. A mobile device to which a secondary battery is applied, such as a laptop computer or a mobile phone, is increasingly used, and to meet the growing demand for performance and a usage time of a mobile device, an output and a capacity of a secondary battery in use is continuously increasing. In this situation, if safety of a secondary battery is not ensured, the secondary battery may be damaged, which may lead to an accident such as an electric shock or a fire and an explosion, causing human life and property losses. Accordingly, various protection devices have been applied or is being attempted to ensure safety of a secondary battery.
Nevertheless, at present, it is difficult to assure that safety of a secondary battery was guaranteed. Particularly, while a secondary battery is in use, gas may be generated in the secondary battery, and in this case, the internal pressure of the secondary battery increases, causing a swelling phenomenon.
The gas generation may occur by various causes such as overcharging or overdischarging, a short circuit, and the like, and is problematic in that it may lead to damage of a secondary battery and even an explosion or a fire. Particularly, when an explosion or a fire occurs to a secondary battery, a serious result may be produced, for example, users of a device to which the secondary battery is applied may be damaged, as well as the device. In addition, when the case 30 of the secondary battery is damaged, an electrolyte solution within the secondary battery leaks out, and as a consequence, damage such as a short circuit or an electric shock may occur. Further, in the case of a hybrid vehicle or an electric vehicle powered by a battery or a power storage system, due to its very high output and capacity, damage caused by internal gas generation may be more serious.